The present invention relates to an ultrasonic diagnostic apparatus which has an array of ultrasonic vibration elements that form an ultrasonic transducer and are excited to generate ultrasonic waves with a predetermined delay time so that ultrasonic beams are deflected.
First, the directivity of an ultrasonic beam emitted from a rectangular ultrasonic vibration element, which is schematically shown in FIG. 1, will be explained. Directivity R of the beam in a remote sound field is given as follows: ##EQU1## where "k" is the number of waves and "b" is the width of the rectangular ultrasonic transducer element.
Directivity R in xz plane is represented by the following equation: ##EQU2## where "2a" is an aperture width along x axis and .gamma.=.pi./2-.alpha..
FIG. 2 shows the directivity of an ultrasonic beam emitted from a rectangular vibration element. From FIG. 1 and equation (2) it is understood that directivity "Rxz" in a remote sound field becomes sharp as the aperture width "2a" increases, assuming that the frequency of the excited ultrasonic wave is constant. It is also understood that directivity Rxz becomes sharp as the frequency increases, assuming that the aperture width is constant. When an ultrasonic beam emitted from the ultrasonic vibration element is deflected or steered with a predetermined time delay, the directivity of the beam causes a sound pressure along the central axis of the element, i.e., the z axis in this case, to decrease. This decrease of sound pressure becomes more noticeable if the frequency of the excited ultrasonic wave becomes higher, the aperture width "2a" becomes greater, or the deflection angle of the ultrasonic wave becomes larger.
Recently, an attention is drawn to an ultrasonic diagnostic apparatus which can utilize high-frequency ultrasonic waves for improvement of the longitudinal and lateral resulutions. In the apparatus, a pulse echo signal reflected from an object is passed through a reception filter to derive a high-frequency component thereof since it is difficult to manufacture an ultrasonic vibration element which can be excited at a high frequency. In the deep depth of the object, e.g., a patient, however, the central frequency of the spectrum of a received ultrasonic pulse signal drops in frequency with depth of penetration. In the conventional ultrasonic diagnostic apparatus, the following measurements are taken for correction. Since the central frequency of the spectrum of an output signal from the reception filter is further decreased in accordance with the depth of the patient, and a sound pressure also progressively decreases toward the deepest region, as shown in FIGS. 3 and 4 respectively, it has been attempted to vary the sensitivity of the signal-receiving circuit in conformity to that depth of the patient. That is, the sound pressure has been corrected by the Sensitivity Time Control (STC) circuit.
However, a decline in the sound pressure arises not only when the ultrasonic beams are attenuated in the deep depth of the patient, but also in the following cases. Each vibration element of the known ultrasonic diagnostic apparatus emits a low-frequency ultrasonic beam. For low-frequency ultrasonic beams, it is unnecessary to take into consideration a decrease in sound pressure, which results from the directivity of the element, even if the ultrasonic beam is deflected, or steered. In the present invention, the ultrasonic vibration element needs to emit an ultrasonic beam of a high frequency and also the ultrasonic beam is deflected. Consequently, a decline in sound pressure cannot be overlooked. FIG. 4 shows the relationship between the depth of a patient with respect to a lateral angle of an ultrasonic beam, and a sound pressure on the central axis of the ultrasonic vibration element. As seen from the above relation, the sound pressure considerably falls at a relatively shallow region of the patient when the frequency of the ultrasonic wave becomes high and the deflection angle of the ultrasonic beam becomes great. When, under such a condition, the B-scan image is displayed on a TV monitor, the displayed image grows darker as the ultrasonic beam is more deflected at a shallow region of the patient.
Moreover there is another problem that the center frequency of the spectrum of the reception filter output drops in frequency with depth of penetration through the patient.